Yay, Instagram! I'm already a happy Instagram user, so I was pleased to see this app included in 23 Mobile Things. I use it to tweak and share photos, as well as post selected photos to Facebook. I sometimes wish it had more filters and frames to choose from (I'd happily pay for such an upgrade), but I find that it's easy to use other apps for missing features and then import the modified photo into Instagram for publishing.
I chose not to download Snapchat for ideological reasons. Its creators have come under fire for providing a false sense of security (i.e., photos are deleted after a length of time) which turned out not to be completely true. There are a number of ways to get around this feature. Added to the fact that Snapchat has been heavily marketed toward teenagers, who are inherently more likely to take photos that they wish to have disappear, I do not wish Snapchat to have my business, even if merely in the form of downloading a free app.